The system mailbox is similar to a post office box: the post office delivers letters addressed to the person who owns that box.
Similarly, the system mailbox is a file where messages are delivered to a particular user. If the file does not exist when mail arrives, it is created. The file is deleted when all messages have been removed.
System mailboxes reside in the /var/spool/mail directory.
Each system mailbox is named by the user ID associated with it. For example,
if your user ID is
karen, your system mailbox is: